One conventional brake system is shown in the following Patent Document-1. The outline of the system will be described. The brake system generally comprises a housing that has therein oil passages through which a brake liquid pressurized by a master cylinder flows, an electric pump that sucks the brake liquid from a reservoir tank provided in the housing, pressure increasing/decreasing electromagnetic valves that are connected to the housing for controlling the brake liquid led to each of wheel cylinders of the road wheels and a control mechanism that controls the pressure increasing/decreasing electromagnetic valves and the above-mentioned electric pump.
The reservoir tank is provided with a cylinder that is formed in the housing and has one end that is connected, as well as to the master cylinder, to one end of a brake liquid passage connected to the corresponding wheel cylinder through the pressure decreasing electromagnetic valve, a synthetic resin piston that is slidably received in the cylinder and biased by a coil spring in a direction to reduce the volume of the cylinder, and a check valve mechanism that is provided at an upper part of the cylinder to control the brake liquid existing between the master cylinder and a liquid pressure chamber of the cylinder.
The check valve mechanism is equipped with a seat member that is formed with a connecting opening through which a brake liquid pressure passage connected to the master cylinder and the cylinder are connected, a ball valve element that is selectively seated on and separated from a valve seat of the seat member to close and opening one side of the connecting opening of the seat member, a valve spring that biases the ball valve element toward the valve seat, and a metal push rod that is lifted up by the piston when the piston is slid upward by a predetermined degree or more by the spring force of the coil spring, so that the ball valve element is separated from the valve seat against the spring force of the valve spring thereby to open one side of the connecting opening.
When, due to operation of the electric pump, the brake liquid in the cylinder is discharged, the piston is moved upward in the cylinder by the spring force of the coil spring thereby pushing up the push rod to separate the ball valve element from the valve seat bringing about an open condition, so that the brake liquid pressure of the master cylinder is led into the cylinder from the brake liquid pressure passage through the connecting opening. While, when the brake liquid pressure is led into the cylinder from the wheel cylinder through the pressure decreasing electromagnetic valve, the piston is moved downward against the spring force of the coil spring suppressing the push rod from pushing up the ball valve element, so that the ball valve element is seated on the valve seat due to the spring force of the valve spring thereby to close the open end of the connecting opening.
However, in the above-mentioned conventional brake system, due to the inherent construction of the check valve mechanism, the inner diameter of the connecting opening of the seat member is larger at a side facing the ball valve element and smaller at a side facing the piston, and thus, at the time when the above-mentioned parts are being assembled, the push rod has to be inserted into connecting opening from the top of the opening, that is, from the upper side where the ball valve element is placed toward (or toward the cylinder) the lower side where the piston is placed. Accordingly, one end of the push rod that is directed toward the ball valve element has a larger diameter and the other end of the push rod that is directed toward the piston has a smaller diameter.